1. Field of the Invention The present invention relates to a method and a device for operating a heatable exhaust-gas sensor.
2. Description of the Related Art
Exhaust-gas sensors such as lambda or NO sensors for example are used for operating internal combustion engines, the sensor signals of which are used to control the internal combustion engine in order to ensure, for example, suitable conditions for an effective emission control in an emission control system.
Particularly exhaust-gas sensors that have an ion-conducting solid electrolyte, such as lambda or NOx sensors for example, require a specific operating temperature in order to achieve the necessary ionic conductivity of the solid electrolyte.
Additionally, the measuring accuracy depends on the temperature of such a sensor. For this reason it is generally necessary to heat the probe and check the temperature and normally to regulate it. To measure the temperature, one normally does without a separate thermal element. Instead, it is possible for example to make use of the highly temperature-dependent internal resistance Ri of the exhaust-gas probe in order to obtain a measuring signal for the sensor temperature. Another measuring signal, which is provided by the heatable exhaust-gas sensor, is for example the Nernst voltage, which allows for an inference as to whether a measuring gas found on the electrodes is thermodynamically balanced.
Due to the comparatively high operating currents of a sensor heater, which is normally operated using a pulse-width modulated voltage, disturbances caused by the pulse edges may occur, which may influence the at least one measuring signal provided by the heated exhaust-gas sensor.
Published German patent application document DE 10 2008 042 268 A1 describes a method for operating a heatable exhaust-gas sensor, in which the detection of the measuring signals is defined as a function of the pulse-width modulated operating voltage of the sensor heater. The detection of the at least one measuring signal is defined as a function of the edges of the pulse-width modulated operating voltage of the sensor heater in such a way that a certain waiting period is specified after the occurrence of a rising or falling pulse edge before the measuring signal is detected.
The present invention is based on the objective of indicating a method and devices for operating a heatable exhaust-gas sensor, in which the sensor heater is operated using a pulse-width modulated operating voltage and in which the at least one detected measuring signal is disturbed as little as possible by the pulse-width modulated operating voltage of the sensor heater.